Sulfidation corrosion-resistant coating containing rare earth metal aluminides



JWIB 1969 M. H. ORTNER ET AL 3, 7,912

SULFIDA'I'IQN CORROSION-RESISTANT COATING CONTAINING RARE EARTH METAL ALUMINIDES Sheet Filed Nov. 2. 1967 FIG. IA

FIG. 13

INVENTORS MARTIN H. ORTNER iheir ATTORNEYS June 1969 M. H. QRTNER ETAL 3,447,912

SULFIDATION CORROSION-RESISTANT COATING CONTAINING RARE EARTH METAL ALUMINIDES Filed Nov. 2. 1967 Sheet 3 or a FIG. 2A

INVENTORS MARTIN H. ORTNER B BY HAROLD M. MCCULLOUGH their Arron/vex;

June 3, 1969 H, QRTNERi ET AL 3,447,912

SULFIDATION CORROSION-RESISTANT COATING CONTAINING RARE EARTH METAL ALUMINIDES Filed Nov. 2, 19s? Sheet 3 of a FIG. 3

FIG. 4

FIG. 5

INVENTORS MARTIN H. ORTN ER a HAROLD M. MQCULLOUGH iheir ATTORNEYS United States Patent US. Cl. 29-1823 9 Claims ABSTRACT OF THE DISCLOSURE As a coating for articles made of the superalloys to protect them from sulfidation corrosion in high temperature, corrosive environments, an alloy of one or more rare earth metals, which may, for example, occur as misch metal, and aluminum. The aluminum is preferably alloyed with the rare earths in a stoichiometric amount to form the intermetallic rare earth aluminide compounds. The coating composition is preferably deposited as a powder layer from a suitable carrier onto the superalloy substrate and is then sintered to form a dense, substantiallyv impervious, bonded coating, preferably under conditions effective to partially diffuse the alloy coating into the substrate to form an adherent metallurgical bond. The coating system may also include chromium, either as a separately deposited layer formed before the rare earth aluminide layer is deposited, or as a co-deposited material in solid solution with the rare earth aluminide, or as an alloy with the rare earth aluminide in the coating composition.

BACKGROUND OF THE INVENTION with engine parts in engine sections operating at high' temperatures, say above 1500 F., thus greatly reducing the fatigue life of the components and considerably shortening the time between engine overhauls.

SUMMARY OF THE INVENTION There is provided, in accordance with the invention, a coating for superalloy substrates that protects them from sulfidation corrosion and provides a solution not only for the specific problem encountered in engines operating in marine environments but also for similar problems in other areas in which sulfidation corrosion of superalloys is encountered at high temperatures.

More particularly, the coating substance of the invention is a rare earth aluminide, such as misch metal aluminide, produced by thermally reacting one or more rare earths with a stoichiometric amount of aluminum to yield a mixture or solution of intermetallic compound aluminides. A powder of the coating material is prepared and deposited on the substrate in a suitable manner, such as by spraying, dipping or, preferably, electrophoretic deposition. A dense adherent coating is then obtained by sintering the layer, preferably under conditions which partially diffuse the coating into the substrate to provide an adherent metallurgical bond at the interface.

The coating system may, if desired, further include chromium, either as a separately deposited layer over which the rare earth aluminide coating layer is later deposited, as a co-deposited material in solid solution with 3,447,912 Patented June 3, 1969 the rare earth aluminide, or alloyed with the rare earth aluminide in a single coating layer.

Although the mechanism by which the rare earth aluminide coatings of the invention resist sulfidation of the substrate is not thoroughly understood, it is believed that the coating layer, or perhaps a thin layer at the interface between the coating and substrate in which partial reaction between the coating compound and the substrate have taken place, is converted under high temperature sulfidation conditions into a stable sulfide compound which acts to prevent diffusion of sulfur into the substrate.

For a better understanding of the invention, reference may be made to the following description of exemplary embodiments, taken in conjunction with the figures of the accompanying drawings, which are photomicrographs of exemplary coatings.

DESCRIPTION OF EXEMPLARY EMBODIMENTS As mentioned above, one exemplary coating composition consists essentially of misch metal (a mixture of rare earths containing major amounts of cerium, lanthanum, neodyminum and praseodymium and minor amounts of other rare earths) and aluminum. The misch metal-aluminurn alloy is prepared by thermally reacting a stoichiometric amount of aluminum with misch metal to yield a mixture or solution of intermetallic compound aluminides, hereinafter referred to as misch metal aluminide.

A suspension of the misch metal aluminide as a fine powder in an appropriate medium is prepared for the purpose of permitting the deposition, such as by an electrophoretic process, of a thin coating of the misch metal aluminide powder onto the substrate. The coatings are then sintered in a neutral atmosphere, such as that afforded by hydrogen, argon or a vacuum, at temperatures of about 1100 to 1200 C. for a time, say two hours, suflicient to produce a dense, substantially impervious, metallurgically bonded coating on the substrate. The temperature and time of sintering should be such that partial diffusion of the coating into the substrate is obtained; in other words, that there should be a slight, minimal chemical reaction between the coating and the substrate. In photomicrographs of misch metal aluminide coatings on L-605 and Hastelloy X," FIGS. 1A and 2A, respectively, the sintered coatings on the samples appear to be composed of two layers. The layer at the interface is apparently a reacted layer in which either the coating is diffused into and has reacted to a minor degree with the substrate or vice versa. This reacted coating layer may provide the ultimate corrosion resistance, inasmuch as numerous instances of coatings having discontinuities extending to the substrate but having no evidence of corrosion have been observed.

Tests have been conducted to determine the relative rates of reaction occurring in the sintering process between misch metal aluminide and three major elements of superalloys, chromium, cobalt and nickel. It was found that misch metal aluminide and chromium were only slightly reactive, while the misch metal aluminide reacted strongly with nickel and cobalt. As a result of these tests it was found that a sintering temperature of 1100 C. was close to the optimum as far as obtaining limited reaction while still securing a good metallurgical bond.

In working with the coating process, it has been found that the sintering operation usually is insufficient to completely sinter the coating material, a powdery outer layer remaining. However, the powdery layer is readily washed from the specimen and a dense adherent inner layer having a thickness on the order of 0.5 to 1.5 mils remains.

In an exemplary series of tests, misch metal containing 53% cerium, 25% lanthanum, 16% neodymium and 5% praseodymium, along with lesser amounts of other rare earths, was thermally reacted with a stoichiometric amount of aluminum to form the misch metal aluminide. Specimens having a coating thickness of 0.5 mil were produced on panels measuring /2" x l" x 0.050" of (a) Inconel 718, (b) Hastelloy X, and (c) L-605 by electrophoretically depositing the prealloyed misch metal aluminide powder and then sintering the coatings in an argon atomsphere for two hours at 1100 C. The coated specimens were embedded in misch metal aluminide as a getter for detrimental impurities in the argon atmosphere during sintering. After sintering, a powdery outer layer remained, but it was readily washed from the specimen leaving a thin, dense layer which was partially diffused into and metallurgically bonded to the substrate.

The specimens were placed in a molten salt bath (65% Na SO -35% NaCl) at 800 C. A specimen of each of the coated substrates was removed at intervals of 19, 44, 72 and 140 hours and each was subjected to metallographic inspection. None of the specimens showed any evidence of attack or penetration into the substrate. However, the metallographic examination revealed occasional cracks in the coating extending to the substrate, but no attack on the substrate was evident, apparently because the diffusion layer inhibited substrate attack.

In a further series of tests, misch metal aluminide powder was electrophoretically deposited to a thickness of 0.5 mil on small panels of (a) L605, (b) Hastelloy X, (c) Rene 41, and (d) B-1900. The specimens were placed in an argon atmosphere at a temperature of 1100 C. for threehours to sinter the coatings. FIGS. 1A and 2A are photomicrographs of the L-605 and Hastelloy X specimens, respectively, as sintered (before corrosion testing).

The specimens were corrosion-tested by suspending them over a molten salt bath (95% Na SO NaCl) and holding them at 900 C. Periodically, a specimen of each superalloy was removed and subjected to metallurgical inspection. As with the first series of tests, the specimens exhibited no evidence of attack or sulfidation into the substrate. FIGS. 1B and 2B are photomicrographs of L-605 and Hastelloy X samples, respectively, after 500 hours of the corrosion tests. The L-60 superalloy (FIG. 1B) appears to have some slight subsurface. disturbances, but the disturbance does not seem to be sulfidation attack. FIG. 2B shows that the Hastelloy X substrate was fully protected and remained unaffected by the severe corrosion test conditions.

In another example, a layer of chromium was electrophoretically deposited onto the substrate and sintered at 1250 C. for six hours in a hydrogen atmosphere. The chromium was then overcoated with misch metal aluminide and sintered at 1100 C. for 3 hours in an argon atmosphere. FIG. 3 shows the structure of this coating after a 500 hour corrosion test at 900 C. over a salt bath (95% Na SO -5% NaCl). No sulfide formation has occurred, nor is there any gross attack at the surface. It appears that the chromium may have completely diffused into the substrate, as evidenced by the wide diffusion band. Also, a large coating discontinuity on the right side penetrates to the reacted coating, but no premature attack is noted around this defect.

Another specimen was prepared by combining misch metal aluminide powder and chromium powder in equal parts by weight, electrophoretically depositing the mixture on a small Hastelloy X panel, and sintering at 1100 C. for three hours. FIG. 4 is a photomicrograph of this test specimen after 327 hours of the molten salt corrosion test at 900 C. The thickness of the coating of this specimen is somewhat greater than the thickness of the specimens described-above and indicates a possibly greater reaction between the substrate and coating composition. There is some evidence of diffusion to the substrate, but no evidence of sulfidation attack. Another specimen was prepared by pre-alloying 75 parts misch metal aluminide and 25 parts chromium, by weight, depositing a powder layer of the alloy mixture on L-605, and sintering in a hydrogen atmosphere for two hours at 1100 C. FIG. 5 is a photomicrograph of the specimen as sintered. Corl rosion tests on the specimen showed good sulfidation resistance for this coating.

The above described embodiments of the invention are intended to be merely exemplary, and those skilled in the art will be able to make numerous variations and modifications of them without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention as defined in the appended claims.

We claim:

1. A combination comprising an article made of a superalloy and a coating composition on the article to protect it from sulfidation corrosion in high temperature, corrosive environments, the coating composition including the reaction product of at least one rare earth metal and aluminum.

2. A combination according to claim 1 wherein the rare earth metal is misch metal.

3. A combination according to claim 1 wherein the aluminum is present in the reaction product in a stoichiometric amount.

4. A combination according to claim 1 wherein the reaction product is partially diffused into the article and at least a portion of the reaction product forms an intermetallic compound therewith.

5. A combination according to claim 1 wherein the coating composition further includes chromium.

6. A combination according to claim 5 wherein the chromium is a separately deposited layer and the reaction product is a layer overcoated on the chromium layer.

7. A combination according to claim 5 wherein the chromium is in solid solution with the reaction product as a co-deposited layer.

8. A combination according to claim 5 wherein the chromium is alloyed with the reaction product.

9. A combination comprising an article made of a superalloy and a coating composition on the article to protect it from sulfidation corrosion in high temperature corrosive environments, the coating composition including the reaction product in powdered form of at least one rare 50 earth metal and aluminum and being sintered on the article to form a dense substantially impervious bonded layer.

References Cited UNITED STATES PATENTS BENJAMIN R. PADGETT, Primary Examiner.

ARTHUR I. STEINER, Assistant Examiner.

US. Cl. X.R. 

